The present invention relates to superhard materials based on cubic boron nitride, said material being used for manufacturing water-resistant inserts for cutting, dressing, and drilling tools, and for wire-drawing dies.
Known polycrystalline superhard materials based on cubic boron nitride can be classified into two groups. In the materials belonging to the first group the crystals of cubic boron nitride are strongly interconnected due to self-binding, i.e. by virtue of diffusion processes taking place in the zone of contact of the particles being sintered, without a binder (accepted Japanese Patent Application No. 49-30357). In the materials belonging to the second group the crystals of cubic boron nitride are interconnected by a binder.
U.K. Pat. No. 990,818 teaches a number of metals such as nickel, chromium, zirconium, cobalt, manganese, copper, rhenium, titanium, and molybdenum as a binder in a polycrystalline superhard material based on cubic boron nitride.
Polycrystalline superhard materials are known which contain, in addition to crystals of cubic boron nitride, crystals of diamond; as a binder use is made of metals (accepted Japanese Patent Application No. 43-30409), refractory materials such as borides and oxides of magnesium and calcium (French Pat. No. 2,201,268).
According to the known methods of producing polycrystalline superhard materials (see, for example, U.K. Pat. No. 990,818), it is possible to produce compact strong polycrystalline materials by sintering particles of boron nitride at high temperatures within the range from 1,200.degree. to 2,400.degree. C. and under pressures above 75 kbar.
Polycrystals produced by the known method exhibit relatively low wear resistance when tested on hardened steels in a cutting tool under dynamic loads (impact strength). This is caused by the fact that no strong bond is formed between the neighbouring crystals of cubic boron nitride in the process of sintering when pure powders of cubic boron nitrides are used.
A method is also known of producing polycrystalline superhard material (accepted Japanese Patent Application No. 43-30409) from a mixture of graphite powder, metallic powder, and crystals of cubic boron nitride under pressures above 50 kbar and at a temperature of 1,200.degree. C. in the region of diamond formation; in this method a binder-metal catalyzes the formation of diamond, carbon-containing material transforms into diamond, particles of diamond and cubic boron nitride are fixed in the binder-metal.
A disadvantage inherent in the above-cited material is its low thermal stability caused by the presence of the remaining reaction products and the binder-metal.
Polycrystals produced by this known method can be used only as abrasives but not as a cutting tool.
Likewise a polycrystalline superhard material is known in the art which is produced from a mixture of powders of cubic boron nitride and boron carbide, the latter component amounting to 25 wt.% in the resulting material (U.K. Pat. No. 975,316). The material is prepared under a pressure equal to at least 15 kbar and at a temperature of 1,050.degree. C.
This known method is disadvantageous in that the resulting material is fragile and therefore cannot be used in a cutting tool under conditions of dynamic loads when working heardened difficult-to-work steels and alloys.
Besides, the known method does not ensure highly uniform material, since simple mechanical stirring of the initial components, cubic boron nitride and boron carbide that have different densities makes it absolutely impossible to obtain strictly uniform distribution of particles of various materials, even in the case when one of the components is taken in a highly dispersed state. Heterogeneity of the material and non-uniform distribution of the components in the bulk of the material deteriorates its mechanical characteristics, in particular, resistance to impact thermal and mechanical loads.